The invention relates generally to a method and apparatus for transporting and erecting a tower at a desired site within a body of water. More particularly, the invention relates to a method and apparatus for rapidly and conveniently transporting, launching, and positioning the tower and for recovering for reuse the portion of the invention employed in transporting the tower.
In the past, offshore towers have been employed advantageously in a number of different marine situations. For instance, offshore towers have been employed as supports for radar or sonar stations, light beacons, and various types of laboratories. Furthermore, offshore towers have frequently been employed in the exploration for oil in an offshore environment.
The use of offshore towers in the exploration for oil has received increasing emphasis as supplies of petroleum indigenous to the major industrial countries have diminished and as countries having significant surplus reserves have become more nationalistic. In the past, the exploration for oil in offshore environments has been conducted in locations having relatively shallow water. Areas of this type which are productive of oil and gas exist, for instance, along the shores of the Gulf of Mexico.
Recently however, dramatic increases in the price of oil have made exploration for oil in the geological strata underlying very deep areas of the oceans economically practical. Indeed, exploration has been conducted to date in water approaching a thousand or more feet in depth. Oil fields submerged in water of this depth may be found for instance along the Pacific Continental Shelf of the United States, certain Arctic regions, and in the North Sea. Exploration of these and other areas having similarly deep water is continuing and, indeed, exploration is being pressed into ever deeper areas of the oceans.
In order to exploit oil fields existing beneath such substantial depths of water, towers formerly deemed quite reliable and effective have undergone drastic redesign to accommodate anticipated prolonged stress of rather high levels, as well as stresses introduced by recurrent natural phenomena. The redesign has resulted in towers enormous in both size and weight. The enormous size and weight of the towers has precipitated great difficulties in the construction, transportation, and erection thereof. The transportation and erection of the towers pose the most difficult problems since these two operations must be conducted on the high seas fully exposed to the effects of what is often a very hostile environment. This is particularly the case with towers which must be erected in areas such as the North Sea.
In dealing with problems regarding the transportation and erection of offshore towers attention has been focused on the concept of a buoyant support which is structurally independent of the offshore tower. Typically, once the tower and buoyant structure reach the desired offshore location, the tower and buoyant structure are disconnected, whereupon the buoyant structure can be employed in connection with other towers. Such an arrangement reduces the weight and surface area of the tower significantly so that not only is the cost of the tower itself reduced, but the tower is rendered less vulnerable to hydrodynamic and seismic forces.
One method and apparatus for transporting and erecting an offshore tower which embraces the concept described in the foregoing paragraph entails the attachment of one or more pontoons to the exterior of the tower. The tower is thereupon floated to a desired offshore location resting upon the pontoons. Once the offshore location is reached, the tower can be erected by releasing the pontoons in a manner causing the offshore tower to settle into an upright posture on the floor of the body of water. This approach has been found to be undesirable in some cases for a number of reasons. For instance, due to the enormous forces of buoyancy exerted in supporting the tower, the pontoons may be subject to sudden, violent movement when released. In addition, if the pontoons are employed to lower the tower into place on the floor of the body of water the center of gravity of the combined tower and pontoon may shift as the pontoons are ballasted. As a result, placement of the tower may be rendered more difficult.
Another method and apparatus for transporting and erecting an offshore tower in which the buoyant support is unitary and can be recovered and reused entails the use of a buoyant structure which is generally rectangular in configuration. The buoyant structure includes two iinterconnected lower pontoons and a pair of upper pontoons connected thereto by vertically extending, buoyant columns. An offshore tower to be transported and erected through the use of this buoyant structure is surrounded by and releasably connected to the upper and lower pontoons and the interconnecting buoyant columns. The tower and buoyant structure are turned upright in the water and separated once the base of the tower is resting on the bottom of the body of water.
Theoretically, such an arrangement has some appeal. However, in practice, significant improvement may be necessary. The structure is relatively complex and requires considerable amounts of materials for construction. The buoyant structure is thus relatively costly. Furthermore, the size and configuration of the buoyant structure may render the combined buoyant support and tower relatively unseaworthy. Furthermore, in light of the manner in which the buoyant structure surrounds the tower, the buoyant structure cannot be rapidly removed. Thus, the entire structure may be subjected to considerable buffeting by winds and heavy seas. Somewhat related to the foregoing problem is that caused by the tendency of the buoyant structure to be hydrodynamically driven into the tower once the connection therebetween is released. Also, the movement of the combined tower and buoyant structure in the course of the erection of the tower may be relatively erratic and sudden, and may for this reason be less suitable.
It would be quite helpful considering the preceding remarks, if a method and apparatus could be provided wherein the buoyant structure requires fewer materials so that the cost may be reduced and the seaworthiness of the structure increased. It would be also desirable if the buoyant structure could be quickly removed from the vicinity of the tower so that the tower is less subject to buffeting and there is less danger of any impacting of the tower by the buoyant structure. It would also be highly useful if a method and apparatus could be provided wherein the combined buoyant support and tower could be moved gently and predictably as the tower is being erected.
Yet another method and apparatus previously used in transporting and erecting an offshore tower is comprised of a pair of pivotally connected barges. As the tower is being conveyed to the desired offshore location, the barges are oriented relative to one another so as to fall in a single plane. One of these barges is considerably larger than the second and can be employed to support the tower as the barges are jackknifed relative to one another to lower the tower to the floor of the body of water. The larger of the two barges remains in a floating condition on the surface of the body of water. Thus, the placement of the tower is dependent upon buoyant structural elements located on the surface of the body of water. This restricts the usefulness of the arrangement described in the preceding to relatively shallow water and towers of relatively small size. It would thus be highly desirable if a method and apparatus could be provided in which the placement of the tower is independent of structural elements located on the surface of the body of water and which can be used to handle large towers in relatively deep water.
A further approach to the problem of transporting and erecting an offshore tower contemplates the use of a conventional barge carrying an inclined skidway built up from the deck and sloping downwardly toward the stern of the barge. Carried atop the inclined skidway is an offshore tower having a central, buoyant column nested among the legs thereof. The offshore tower is transported atop the skidway and once the desired offshore location is reached is disconnected from the barge and moved down the skidway in response to forces exerted by an auxiliary vessel. The auxiliary vessel ultimately pulls the offshore tower into the water where the tower assumes a horizontally floating posture. Thereafter, the central bouyant column is ballasted to turn the tower upright and lower it to the floor of the body of water.
Though such an arrangement may be suitable in certain circumstances, a number of serious structural problems may be involved. For instance, the use of an inclined skidway requires the fabrication of an additional structural element as an integral part of the bouyant support. Furthermore, the skidway must be of a highly durable character, capable of sustaining towers of enormous weight. This sort of structure necessarily requires considerable amounts of materials and labor. Additionally, the structure of the inclined skidway may render the barge somewhat top heavy and thus may diminish the degree of seaworthiness of the combined barge and offshore tower. The requirement of an inclined skidway may also render the barge less flexible in use, as towers of different configuration may require skidways of different configuration. Additionally, and quite importantly is the problem caused by the required length of the skidway. Because the skidway extends along most of the length of the tower, considerable resistance may be offered against the movement of the tower down the skidway and into the water.
Other problems reside in the control of the movement of the tower as it is launched. It is important that the movement of the offshore tower be carefully controlled as the tower enters the water as well as when it is being turned upright. The movement of the offshore tower down the skidway, into the water off the stern of the barge, takes place with the tower sliding in a direction roughly parallel to the longitudinal axis thereof. Since only a small portion of the tower, viz. the base, enters the water first, undesirable movement of the tower may occur. There may not be sufficient initial positive control of the movement of the tower for safe and reliable erection thereof on the floor of the body of water.
Perhaps the most significant problem of the approach presented above resides in the fact that the launching of an offshore tower end first from a barge by sliding the tower down an inclined skidway may greatly alter the posture of the barge in the water. The stern of the barge may be forced into an essentially submerged condition. Once the tower clears the barge however, the barge may undergo rapid, violent movement in response to buoyant forces exerted on the stern or other areas of the barge undergoing inordinant amount of deflection.
Considering the foregoing remarks, it would be highly desirable if a novel method and apparatus for transporting and erecting an offshore tower could be provided in which no additional structural elements are required for the purposes of launching the tower and in which the bouyant structure employed to transport the tower is readily adaptable to various structures of different configuration. It would be desirable as well if resistance to the launching of the tower could be reduced or substantially eliminated. It would also be highly desirable if in launching the tower the movement thereof could be positively controlled to within relatively close tolerances. It would also be desirable to avoid violent movement of the bouyant support.
Other methods and apparatus for transporting and erecting a tower employed in the prior art also do not afford sufficient control over the movement of the tower. Control of the movement of the tower involves two distinct aspects. The first has to do with the particular manner in which the tower is first introduced into the water. In many instances in the prior art, the tower is introduced into the water in such a way that the tower may be subjected to undesirable movement. Clearly, if such movements cannot be adequately controlled, erection of the tower may be greatly hampered. A second aspect of the problem resides in the control of the tower once the tower is in the water and as it is being turned upright prior to placement on the floor of the body of water. As the tower is being turned upright, the tower may be vulnerable to rolling. This mode of uncontrolled movement also can render placement of the tower difficult. It would therefore be highly desirable if a novel method and apparatus could be provided in which the movement of the offshore tower can be controlled to an extent sufficient to prevent the tendency of the tower to pitch or roll from interfering with the placement of the tower.
The problems suggested in the preceding, while not exhaustive, are among many which tend to reduce the effectiveness and desirability of methods and apparatus of the prior art for transporting and erecting offshore towers. Other noteworthy problems may also exist; however, those presented in the discussion above should be sufficient to demonstrate that such methods and apparatus appearing in the prior art have not been entirely satisfactory.